1. Field of the Invention
The present invention relates to an optical pickup device, and particularly to a reading optical system of an optical pickup device.
2. Description of the Related Art
The recording capacity of a single layer in an optical disc largely depends on the wavelength of a semiconductor laser to be used and the numerical aperture (NA) of an objective lens. The shorter the wavelength of a semiconductor laser or the larger the NA, the higher the recording density can be made. Thus, the capacity of an individual layer can be increased. The principal optical disc drives currently on the market are DVD (Digital Versatile Disc) drives that use red light with a wavelength near 650 nm and an objective lens with an NA of 0.6. Meanwhile, an optical disc drive having a recording density higher than that of a DVD drive is also delivered on the market. Such an optical disc drive uses a blue-violet semiconductor laser with a wavelength near 405 nm as a light source and an objective lens with an NA of 0.85. A conceivable way to further increase recording density beyond the recording density currently achieved is to further shorten the wavelength to be used. However, it is expected to be difficult to develop such a semiconductor laser of the ultraviolet region, which is of wavelengths shorter than those of the blue-violet region. Moreover, with regard to increasing the NA of an objective lens, the improvement in recording density by means of increasing the NA of an objective lens is also difficult because the NA of the objective lens in air is 1 at most.
In such a circumstance, a double-layer structure has been employed as a way to increase the capacity of a single optical disc. Non-Patent Document 1 proposes a technique of a double-layer phase-change disc. When a double-layer optical disc is irradiated with a laser beam, a problem of crosstalk between layers arises, because adjacent layers are irradiated simultaneously with the laser beam. A measure having been taken to suppress this problem is to increase an interval between layers. Since a laser beam is focused on a layer intended to be irradiated (hereinafter, a target layer), the focal position of the laser beam deviates from a layer other than the target layer. Thereby, the crosstalk can be reduced.
However, increase in the inter-layer interval leads to the problem of spherical aberration. A recording layer is embedded within a polycarbonate material having a refractive index different from that of the air. The spherical aberration varies depending on the depth from the disc surface. An objective lens is designed in a way that its spherical aberration is made small only for a particular layer. Accordingly, spherical aberration increases when the focal point of a laser beam is shifted to another layer. This is because the distance from the disc surface to the focal position is different between the layers. This aberration can be corrected by setting an expander lens optical system usually formed of two lenses or a liquid crystal element in front of the objective lens. Specifically, the aberration can be corrected by changing the distance between the two lenses or the phase of the liquid crystal element. However, it is difficult to correct large spherical aberration, in view of the range of spherical aberration compensable by the liquid crystal element or the need to realize a lens moving mechanism within a small-sized optical disc drive apparatus.
When a multi-layer structure is employed to increase the recording capacity, a larger number of layers should be placed at narrower intervals because the correctable limit of spherical aberration restricts the total thickness of the multiple layers. For this reason, a problem of inter-layer crosstalk remains in a practical optical drive apparatus for multilayer optical discs.
In order to reduce such crosstalk, Non-Patent Document 2 describes the use of a feature in which, when reflected light from a multi-layer optical disc is focused by a lens, the focal positions of reflected light beams from a target layer and an adjacent layer thereof are different from each other on an optical axis. Specifically, a grating is disposed in such a manner as to include the optical axis, and a reflecting mirror is disposed at the focal position of the reflected light beam from the target layer. The reflected light beam from the adjacent layer irradiates the grating, and accordingly is attenuated. Meanwhile, the reflected light beam from the target layer is transmitted through a gap between the grating and the reflecting mirror, and accordingly can return to a detection system without being attenuated. In this manner, the inter-layer crosstalk can be reduced.    [Non-Patent Document 1] Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 956-960    [Non-Patent Document 2] ISOM/ODS' 08, Technical Digest Post-deadline Papers, TD05-155 (2008)